The purpose of this investigation is to assess the role of prostaglandins in macrophage activation. We have shown that resident or elicited mouse peritoneal macrophages cultured under lipopolysaccharide (LPS)-free conditions do not destroy tumor cells as assessed by tritiated thymidine release. Incubation of these macrophage monolayers for 24 hours resulted in an increase in their cytolytic activity from 1-4% to 10-44%. LPS treatment of elicited macrophages resulted in an increase in cytolytic activity which was inhibited by indomethacin. The tumoricidal activity directly correlated with the PGE2 produced by the macrophages. Treatment of these macrophages with PGE2 alone also resulted in their activation. Lymphokine (MAF) treatment resulted in activation of elicited macrophages without an increase in PGE2. However, the concentration of PGE2 in MAF paralleled the capacity of the MAF to activate elicited cells. Treatment of resident macrophages with PGE2 before lymphokine resulted in an augmentation in their response to MAF. These results suggest that activation of resident and elicited macrophages by LPS appears to be mediated by PGE2 and that PGE2 may prime cells for activation by MAF.